The present invention relates to dampening agents or dampening agent concentrates for offset printing. In the course of applying printing ink to the printing plates enclamped on the printing cylinders, offset printing needs so-called "dampening water" in order to keep the non-image areas on the printing plates free from printing ink. Herein it is necessary for the quality of the print that the dampening water or agent thoroughly wet the non-image areas on the printing plates and, furthermore, become properly emulsified with the printing ink applied in the image areas. Such dampening agents for offset printing essentially consist of water, to which have been added customarily low amounts of modifying additives such as pH-regulating substances, humectants, wetting agents, film formers, preservatives and also, if desired, water-miscible organic solvents.
The dampening water is customarily prepared before printing by the printer himself. The modifying additives being in the meantime present predominantly in the form of premanufactured dampening water additives or dampening agent concentrates available from the relevant specialist suppliers, and need only be added to water or be diluted with water.
It is predominantly an essential requirement for dampening agents which are to be used in offset printing that they have a stable pH value in the acid range, for example about pH 3-6, but usually about pH 4-5.5. This is normally achieved by buffer systems based on polycarboxylic acids or polyhydroxycarboxylic acids and salts thereof. The most frequent buffer system is sodium citrate/citric acid.
However, such dampening agents can lead to corrosion phenomena on the printing machine parts with which they come into contact, in particular on the printing cylinders, which are predominantly made of nickel metal or have a nickel coating. Herein the corrosion problems can range from the dissolving of nickel metal on the cylinder surfaces to serious cases of pitting, predominantly in the contact areas between printing plates and printing cylinders. The consequences are dimensional changes which can have an adverse effect on the quality of the printed product, shorter operating lives of the printing cylinders, and also reduced runs, ranging to serious damagetto the printing machines.
There has accordingly been no shortage of attempts to reduce the corrosive properties of the dampening agents by means of various corrosion inhibitors. For instance, corrosion inhibitors have been added to the dampening agents such as, for example, organic zinc salts such as zinc gluconate and zinc glucoheptonate, water-soluble inorganic polyphosphates, phenylarsonic acid and derivatives thereof, propargyl compounds, thiourea and derivatives thereof. According to EP No. 108883, the addition of 1H-benzotriazole is supposed to have a marked corrosion-inhibiting effect. However, as stated there and as confirmed in in-house experiments, the inhibitor action of 1H-benzotriazole is only acceptably adequate in the relatively weakly acidic medium around pH 5 and higher, but is insufficient in the case of dampening agents at a lower pH value. All the known corrosion inhibitors are thus known to have in common that, although they offer a certain degree of corrosion protection, they do not inhibit corrosion completely, in particular not over a wider pH range. This is in particular also true when the dampening agents have been made up with demineralized water and have a pH value of distinctly below 5. There thus continues to exist in the printing industry a particular need for dampening agents for offset printing which have at least distinctly reduced corrosive properties.